1. Field of the Invention
The present invention generally relates to a technology for wireless transmission, and particularly relates to a mobile station, a base station, and a program for and a method of wireless transmission.
2. Description of the Related Art
The development of a fourth-generation mobile communications method, which is the next-generation mobile communications method beyond IMT-2000 (International Mobile Telecommunications-2000), has been underway. A fourth-generation mobile communications method flexibly supporting a multi-cell environment including a cellular system, as well as an isolated cell environment such as a hot-spot area and an indoor environment so as to seek a further increase in spectral usage efficiency at the respective cell environments, is being desired.
As a candidate wireless access method in the fourth-generation mobile communications which is applied to a link from a mobile station to a base station (referred to as an uplink below), DS-CDMA (Direct Sequence-Code Division Multiple Access) is considered to be promising from the point of view that it is particularly suitable for the cellular system. DS-CDMA multiplies to a transmitting signal a spreading code so as to spread to a wideband signal for transmission (refer to below-identified Non-Patent Document 1, for example).
Reasons that DS-CDMA is suitable for the multi-cell environment including the cellular system are described below.
First, a suppression of the peak-to-average power ratio to a low level relative to a wireless access method using a large number of sub-carriers such as OFDM (Orthogonal Frequency Division Multiplexing) and MC-CDMA (Multi-Carrier-Code Division Multiple Access) is enabled. Therefore, it is easy to implement a reduced power consumption which is one of the important desired conditions for the mobile station.
Second, while there is potential for a reduction in a required transmitting power by a coherent-detection using dedicated pilot channels, assuming that power levels of the pilot channels are the same, DS-CDMA relative to such methods as OFDM and MC-CDMA has a larger pilot-channel power per carrier. Therefore, an accurate channel estimation so as to suppress the required transmitting power to a low level is enabled.
Third, in the multi-cell environment, even when using carriers having the same frequency in neighboring cells, DS-CDMA enables a suppression in interference from the neighboring cells (referred to as “other-cell interference” below) due to a spreading gain obtained by spreading. Therefore, an easy implementation of one-cell frequency reuse which allocates all available spectral bands to the respective cells is enabled. Therefore, relative to TDMA (Time Division Multiple Access) which divides all available spectral bands so as to allocate different spectral bands to the respective cells, an increase in the spectral usage efficiency is enabled.
However, as DS-CDMA is a wireless access method suitable in the multi-cell environment, there is a problem as described below as a cause for concern. In other words, in the isolated cell environment such as the hot spot area and the indoor environment in which an effect of other-cell interference is usually small, the advantage of reducing other-cell interference by spreading is low. Therefore, in DS-CDMA, a large number of signals of simultaneously accessing mobile stations need to be accommodated in order to achieve the same level of spectral usage efficiency as in TDMA.
For example, when the respective mobile stations transmit transmitting signals having multiplied spreading codes with spreading factor of SF, the transmission data rate becomes 1/SF so that with DS-CDMA there is a need to accommodate the signals from SF mobile stations in order to achieve the same level of spectral usage efficiency as TDMA. However, in an actual uplink wireless propagation environment, an effect of Multiple-Access Interference (MAI) in which the signals from the respective mobile stations interfere with one another due to differences in condition of propagation from the respective mobile stations to the base station (for instance, propagation delay time, change of propagation channel) becomes predominant. As a result, the spectral usage efficiency normalized by the spreading factor as described above is reduced to about 20-30 percent.
On the other hand, IFDMA (Interleaved Frequency Division Multiple Access) is being studied as a wireless access method which enables a reduction of the MAI as described above (for example, refer to below-identified Non-Patent Document 2). IFDMA applies a symbol repetition to a data symbol so as to perform a sorting to generate a predetermined symbol pattern and to multiply a mobile station-specific phase to a transmitting signal for transmission. IFDMA reduces the MAI as the generation of the predetermined symbol pattern and the multiplying of the mobile station-specific phase set the signals from the respective mobile station to be arranged on the frequency axis without overlapping one another.
Furthermore, a study of a transmission timing control as another method of reducing such MAI so as to improve the spectral usage efficiency is underway (for example, refer to below-identified Non-Patent Document 3). FIG. 43A and FIG. 43B are diagrams which respectively illustrate time charts for a case of applying a transmission timing control in an uplink and for a case of not applying such control according to the related art. As illustrated in the case of FIG. 43A in which a transmission timing control is not applied, the signals transmitted from the respective receivers 200 through 220 have non-coincident received timings at the base station 100 due to the different delay times of propagation to the base station 100. Therefore, with the transmission timing control, the transmitting timings of the respective mobile stations 200 through 220 are controlled so that the respective signals transmitted from the respective mobile stations 200 through 220 are received at the same timing at the base station 100. Such performing of transmission timing control enables reception of signals at the base station 100 from the respective mobile stations 200 through 220 at the same time (refer to FIG. 43B). When using at this time orthogonal code as spreading code, the received signals from the different respective mobile stations at such timing are orthogonal to one another so as to reduce the MAI. Hereby, improvement in the spectral usage efficiency is enabled.
Furthermore, a study of a technology which suppresses, for a received signal affected by multi-path interference, multi-path interference by signal processing at the receiver is underway. A multi-path interference canceller (for example, refer to below-identified Non-Patent Document 4) as illustrated in FIG. 44, a chip equalizer (for example, refer to below-identified Non-Patent Document 5) as illustrated in FIG. 45, and a frequency-domain equalizer (for example, refer to below-identified Non-Patent Document 6) as illustrated in FIG. 46 are representative examples.
The multi-path interference canceller as illustrated in FIG. 44 estimates and generates at a multi-path interference signal estimator 351 a signal component causing multi-path interference (referred to as a multi-path replica below) and subtracts the estimated multi-path interference replica as described above from a received signal. Hereby, a reproduction of a received signal having a reduced multi-path interference effect is enabled.
The chip equalizer as illustrated in FIG. 45 generates at a channel-matrix generator 361 a channel matrix which shows the amount of change through a propagation channel of a received signal so as to derive from the matrix at a weighting factor estimator 362 a weighting factor which reduces multi-path interference from the matrix and to multiply at the chip equalizer 363 the weighting factor as described above and the received signal (this operation is referred to as an chip equalization). Hereby, a reduction of an effect of multi-path interference is enabled.
The frequency-domain equalizer as illustrated in FIG. 46 converts a received signal at a time-to-frequency converter 371 into a frequency-domain signal so as to then derive at a weighting-factor estimator 372 a weighting factor which reduces multi-path interference, and to multiply at the frequency-domain equalizer 373 a weighting factor to the received frequency-domain signal so as to convert to the time domain at the frequency-to-time converter 374. The performing of such operations enables a reducing of the effect of multi-path interference.
Non-Patent Document 1
H. Atarashi, S. Abeta, and M. Sawahashi, “Broadband packet wireless access appropriate for high-speed and high-capacity throughput,” IEEE VTC2001-Spring, pp. 566-570,
May 2001
Non-Patent Document 2
M. Schnell, I. Broek, and U. Sorger, “A promising new wideband multiple-access scheme for future mobile communication systems,” European Trans. on Telecommun. (ETT), Vol. 10, No. 4, pp. 417-427, July/August 1999
Non-Patent Document 3
Een-Kee Hong, Seung-Hoon Hwang, and Keum-Chan Whang, “Synchronous transmission technique for the reverse link in DS-CDMA terrestrial mobile systems,” pp. 1632-1635, Vol. 46, No. 11, IEEE Trans. on Commun., November, 1999
Non-Patent Document 4
Kenichi Higuchi, Akihiro Fujimura, and Mamoru Sawahashi, “Multi-path Interference Canceller for High-Speed Packet Transmission With Adaptive Modulation and Coding Scheme in W-CDMA Forward Link,” IEEE Selected Area Communications, Vol. 20, No. 2, February 2002
Non-Patent Document 5
A. Klein, “Data detection algorithms specially designed for the downlink of CDMA mobile radio systems”, in Proc. IEEE VTC'97, pp. 203-207, May 1997
Non-Patent Document 6
D. Falconer, SL Ariyavisitakul, A. Benyamin-Seeyar, and B. Eidson, “Frequency domain equalization for single-carrier broadband wireless systems”, IEEE Commun. Mag., Vol. 40, No. 4, pp. 58-66, April 2002.
However, because there is no spreading gain in IFDMA, it is necessary to divide all available spectral bands so as to allocate different spectral bands to the respective cells. Therefore, even when adopting such wireless access method, it is difficult to seek an increase in the spectral usage efficiency in both the multi-cell environment and the isolated cell environment. The increase in the spectral usage efficiency increases the number of mobile stations which can be accommodated in the base station at the respective cells so as to achieve an increased communications-link capacity.
Furthermore, as the related-art technologies as described above are technologies concerning single elements within a wireless transmission system, in order to actually build a wireless transmission system, a study on a specific configuration of a base station and a mobile station as well as on an overall configuration and also on a specific controlling method of these single-element technologies is needed.
However, as a sufficient study concerning the points as described above has not been performed to date, there is a demand for a specific configuration of a base station and a mobile station.